Play of the game

This is a minimal description of the game in general, with elements common to all or almost all variants of the game. For more specific rules, see each code's individual articles.
Prior to the start of a game, a coin toss determines which team will kick off the ball to their opponent. Each team lines up on opposite halves of the field, with a minimum ten yards of space between them for the kickoff.
At this point, play from scrimmage begins. The team in possession of the ball is on offense and the opponent is on defense. At least half of the players (seven in standard American and Canadian football) on the offense must line up on the line of scrimmage, including the snapper, who handles the ball before play commences; the rest must line up behind the line. Neither the offense nor the defense can cross the line of scrimmage before the play commences. Once the offense sets in formation, the snapper snaps the ball to one of the players behind him. The play has now commenced, and the offense's goal is to continue advancing the ball toward their opponent's end zone. This can be done either by running with the ball or by a rule unique to gridiron football known as the forward pass. In a forward pass, a player from behind the line of scrimmage throws the ball to an eligible receiver (another back or one player on each end of the line), who must catch the ball before it touches the ground. The play stops when a player with the ball is tackled to the ground, runs out of the boundaries of the field, or a forward pass hits the ground without being caught. At any time, the player with the ball can attempt a backward, or lateral, pass to any other player in order to keep the ball in play; this is generally rare. In order to keep play moving, the offense must make a certain amount of progress (10 yards in most leagues) within a certain number of plays (3 in Canada, 4 in the United States), called downs. If the offense does indeed make this progress, a first down is achieved, and the team gets 3 or 4 more plays to ac ieve another 10 yards. If not, the offense loses possession to their opponent at the spot where the ball is. More commonly, however, the team on offense will, if they have a minimal chance of gaining a first down and have only one play left to do it (fourth down in the U.S., third down in Canada), attempt a scrimmage kick. There are two types of scrimmage kick: a punt is when the ball is kicked downfield as far as possible; the kicking team loses possession of the ball after the kick and the receiving team can attempt to advance the ball. The other scrimmage kick is a field goal attempt. This must be attempted by place kick or (more rarely) drop kick, and if they pass through the goal set at the edge of the opponent's end zone, the team scores three points (four in rare variants and special circumstances).
If the team in possession of the ball, at any time, advances the ball into the end zone, it is a touchdown, and the team scores six points and a free play known as a try. In a try, a team attempts to score one or two points (rules vary by each league, but a field goal on a try is usually worth one point while another touchdown is worth two).
If the player with the ball is tackled while he is in his own end zone, or if a team throws or fumbles the ball out of its own end zone, the defense scores a safety, worth two points.
After a try, safety or field goal, the team that had possession of the ball goes back to the middle of the field and kicks the ball off to their opponent, and play continues as it did in the beginning of the game.
Play continues until halftime. (Each team switches their side of the field with the other halfway through each half, at the end of a quarter.) After the halftime break, a new kickoff occurs. Whichever team has more points at the end of the game is declared the winner; in the event of a tie, each league has its own rules for overtime to break the tie. Because of the nature of the game, pure sudden-death overtimes have been abolished at all levels of the game as of 2012.